It took a long time for nations to set a speed limit on the road to a warming world.

But for the past four years, even though negotiators have never arrived at a plan for avoiding dangerous climate change, they have agreed on a goal: limiting the increase in the Earth's global average surface temperature to 2°C (3.6°F) above the preindustrial level.

"A policy narrative that continues to frame this target as the sole metric of success or failure to constrain climate change risk is now itself becoming dangerous," wrote Todd Sanford and Peter Frumhoff of UCS in the commentary published Wednesday in Nature Climate Change. "[It] ill-prepares society to confront and manage the risks of a world that is increasingly likely to experience warming well in excess of 2°C this century," said the piece, co-authored by Amy Luers of the San Francisco-based Skoll Global Threats Fund, and Jay Gulledge, of the U.S. Department of Energy's Oak Ridge National Laboratory. (See a blog on the commentary by Gulledge.)

Rethinking the Target

The authors are by no means the first to suggest a rethinking of the 2°C goal. Todd Stern, the lead U.S. climate negotiator in President Barack Obama's administration, provoked anger in 2012 when he said a more "flexible, evolving" approach might be more effective in spurring a political accord. Coming at the issue from an entirely different angle, retired NASA climate scientist James Hansen and a group of colleagues wrote in December the 2°C target was not stringent enough, and "so dangerous" as to be "foolhardy." At that level, the world risked initiating feedbacks in the climate system, such as the melting of ice sheet area, that could trigger irreversible warming out of humanity's control.

Hansen and colleagues suggested a 1°C target was far less dangerous. The Earth has warmed 0.85°C from 1880 (preindustrial times) to 2012, according to the latest consensus science reported in September by the Intergovernmental Panel on Climate Change (IPCC), the scientific body established by the United Nations to inform governments of climate risks.

The UCS scientists and colleagues took the IPCC to task for issuing reports that present different future scenarios, while making no judgment on the relative likelihood of the varying projections, "implicitly treating all scenarios as equivalently plausible."

"Inadvertently, the [IPCC] reinforces the present narrative by failing to provide policymakers with guidance on how to weigh the relative likelihood of the scenarios of future concentrations of heat-trapping gases and other drivers of warming on which its climate change projections are based," the authors said. (See related, "UN Climate Report Relevance Debated Amid Rollout.")

Gavin Schmidt, a climate scientist with the NASA Goddard Institute for Space Studies, agreed that the world needs to assess the risks of high-magnitude warming, but he said criticism of the IPCC is misplaced.

"We do need to assess the risks of high scenarios, not just assume that [2°C] guiderail is achievable, but the call for IPCC to give probabilistic information about different [scenarios] is just a non-starter," Schmidt said in an email. "What is the probability of an international carbon tax? A breakthrough in nuclear power generation? Solar? Of missing feedbacks in models becoming important? These are undefinable, and yet essential for what they are calling for.

"We need instead to work around these limitations, not pretend they can be vanished away by an IPCC statistician's pen," Schmidt said.

Andrew Jordan of the Tyndall Centre for Climate Change Research in Norwich, England, co-author of a similar call for a reassessment of the 2°C target, said the IPCC actually "suggested" in its September report that the 2°C target would be breached. The IPCC report showed that the world's "carbon budget," the amount of greenhouse gas that can be emitted without exceeding 2°C, could be used up entirely by 2040, Jordan noted.

"Given where emissions are in 2014, now is, as the authors of this new paper argue, precisely the time to re-think the current policy narrative," Jordan said in an email. "At present, policy makers are stuck in a binary debate about whether or not the target of [2°C] should or will be met."

Jordan noted there are risks in focusing a new debate on the 2°C target. "If the target is downplayed and/or higher temperature limits are debated, breaching the [2°C] ceiling could easily become a self-fulfilling prophecy," he said. "But failing to debate it could mean that by 2020, international policy is premised on an unrealistic target, further undermining its credibility and under-preparing the world for the challenge of adapting to higher temperatures."

Prepare for the Worst

Climate scientists first proposed the 2°C target in the mid-1990s as a way of giving substance to the commitment nations had made to address climate change at the 1992 Earth Summit at Rio de Janeiro. It was not until 2009 in Copenhagen, after the 15th annual session of negotiations around that goal failed to reach a binding treaty, that nations instead signed on to an accord pledging to work toward the 2°C limit.

Since then, the UCS scientists and colleagues wrote in their new commentary, "the foundation on which the 2°C target was built has steadily eroded." Not only have global carbon emissions continued to rise 3 percent a year, but the science has made more clear that human populations and natural systems face serious risk of substantial climate damage at warming less than 2°C, they said.

The scientists didn't spell out a different target in the new commentary, but instead, a different approach. They endorsed an idea borrowed from national security and defense planning, a framework for "climate security," that has been proposed by scientists at the London-based nonprofit EG3. This "ABC" approach says policymakers should aim for an "ambitious" target for reducing carbon emissions, while "building" for, or adapting for, greater warming than targeted, and engaging in "contingency" planning for future climate emergency.

The authors said it might also motivate "difficult but much-needed dialogue and planning" for the drastic measures needed to address "truly disruptive impacts." Two examples: relocation of development from floodplains around London after 2060 and the creation of water-efficient corn varieties for Africa, would require planning and investment now, they said. (See related "Quiz: What You Don't Know About Food, Water, and Energy.")

"Calling for swift and deep reductions in emissions, although essential, is not sufficient," said the scientists. "Confronting and managing the risks of high-magnitude warming will require a science-based policy narrative that honestly communicates these risks, accounts for potential policy failures and climate emergencies that may occur, and helps society weigh the adoption of mitigation and adaptation options that themselves pose significant risks, costs and uncertainties."

Who caused all the climate change before humans? I mean "science" tells us of all the ice ages well it had to warm back up right? Maybe we're still warming from the last ice age. Honestly I think its very vain of us to think that we can control the earths temperature by exhaling more... that causes CO2 last time I checked. So the more we exhale the worse off the earth is, just don't ask the plants who seem to be in favor of CO2.

Didn't Al Gore say that by 2014, New York City would be under water? The sea level is still about the same as it was when his prediction was made. What WOULD HAPPEN if the average world temperature increased 2C or 5C or whatever? Well, the atmosphere would hold more water, in fact a lot more water. The polar areas would still be cold enough to freeze water, so more snow would accumulate. The increased downward pressure from more snow would cause the ice to flow faster from the ice caps. This MAY tend to stabilize the system, or ironically, the earth may slip into an ice age and the sea level would fall.

What I'm trying to say is that I don't believe temperature fluctuations would have much of an impact. The polar areas would still be cold. If the average temperature in Antarctica went from say, -30F to -23F, so what?

A large meteor impact would obviously be disastrous but the system would eventually stabilize. If the Atlantic-Mediterranean connection were to close, as it has a few times in the past, the Mediterranean would dry up and MAYBE cause a major ice-age. If Yellowstone blew, we'd all be screwed. Same, if God decided to end His experiment (Yes, I do believe God exists, but I am not religious and don't belong to any religion). And don't get me started on the evolution scam! I will write my feelings about that another day. As a geologist and computer programmer, and a person with a lot of common sense, I am certain that all the "scientists" that push the evolution agenda are dead wrong.

So average temperature increases have consistently fallen beneath IPCC estimates for a decade and a half, yet somehow we're supposed to still fall for the constant, predictable refrain of: "it's even worse than we thought".

Look at the capital infrastructure that is being "invested "in at this time. Where is it going ? Mainly to fossil fuel dependent industries. These will require a long life to write down. Also, there are various countries that operate as fossil fuel companies and need that income to survive. How are they going to close shop and maintain their government budgets?

Sorry, I understand the science and do not see how we will stop emissions.

I'm always just completely amazed at people on any of the many websites who write and post so often vehemently deny that GW is human caused. (Including the various other non human GW justifications.) The ideology, denial and fear is obvious, yet still I'm amazed at this willful ignorance. For instance there has been no reason for scientists directly involved with climate research to falsify their results for profit... Science is the only way to know what is happening. Yet you have those who say science itself is not to be trusted and those who are simply ideologically opposed so impossible to change the narrative.

So why do 97 percent of climate scientist as well as most others who actually have the credentials to say the debate is over....and so many believe otherwise? (Not sure how many as they are a vocal minority.) Yet then those who deny are almost always those who only get their info from the propaganda machine yet also willfully take the message from this machine and someone feel less fearful or what. I have a hard time with this mental state.

Obviously corporations don't want to change business-as-usual and many including the Koch Brothers spend significant money to further this propaganda. Also there is the ideological divide that keeps so many like the deniers here actively engaging as, bluntly, dupes in the propaganda machine. If you don't like Al Gore then that is enough. Yet there is also the fear factor. For many it is just too much to imagine that humans are significantly effecting the atmosphere and the dire consequences.

Personally I can easily see that we treat our (and planet in general) air as a vast waste dump. Just the current level of CO2 is proven to come from human activity and that is all anyone should need to be convinced. Even with this irrefutable proof, how can anyone believe this is not a problem? It truly blows my mind.

Perhaps humanity is on the cusp of a paradigm shift, or coming out of a immature mindset to one of "transcendental" reality. I like to think that once it becomes obvious, when there is real economic damage that effects everyone hard enough and directly enough then humanity will wake up and start actually living properly rather than excist in a fog of consumption and drudgery.

So for now those who are not afraid of confronting the information will just have to wait and hope reason overcomes fear and it's related greed and willful ignorance.

The hour is much later than most people realize. 2°C was never put forward by climate scientists . It came from an economist, William Nordhaus on behalf governments and industry. In 1990 climate scientists at the United Nations Advisory Group on Greenhouse Gases said 1°C could be disastrous, but were pressured to agree to 2°C for economic reasons.

Yes, Dr. Schmidt, the present probability of getting a world-wide carbon tax, preferably connected with a dividend mechanism, is low, but are we to sit by and let business-as-usual events continue to the point of an irreparable accelerating situation in which we continue to degrade our atmosphere?

Those who produce carbon-based fuels should be taxed at the source (oil/gas wellhead, coal mine, ports of entry, imports and exports). The tax should be calculated on potential CO2 and be gradually increasing on a yearly basis. 100% of the monies collected should be returned to the citizens.

The carbon-based fuel producers should be made to become deeply involved in the development of renewable energy systems, including wind, solar, water, geothermal, and even nuclear, given that the latter can be rendered safely made and the waste safely stored. We will need all these forms of energy production. All citizens, indeed all nations, should join the effort to mitigate climate change. Whatever is determined to be the temperature goal, this is a world-wide problem and, therefore, must be assumed on a world-wide level. In my view, doing nothing is the height of negligence. We desperately need an ecological economics.

" Climate scientists first proposed the 2°C target in the mid-1990s as a " This is incorrect. The 2C target was first introduced by an economist. It is totally incompatible with civil society. In fact we will struggle to survive at 1C+. The current onslaught of extreme weather and ocean acidification is already enough to cripple global civilization and we are already sliding down the cliff. There is I am afraid no way back now.

" Climate scientists first proposed the 2°C target in the mid-1990s as a " This is incorrect. The 2C target was first introduced by an economist. It is totally incompatible with civil society. In fact we will struggle to survive at 1C+. The current onslaught of extreme weather and ocean acidification is already enough to cripple global civilization and we are already sliding down the cliff. There is I am afraid no way back now.

Climate scientists say "could be" because there is no way to kow exactly how much CO2 we'll emit in the next years, and no way to know exactly climate change events that glbal warming will cause. But we know we are headed for a 5C global temperature increase in eighty years according the IPCC is we keep burning carbon at our present rate of 37 trillion tons of excess CO2 a year.

If you are going to tell billions of innocent children to believe that science "believes" that the end is near, you should know that the scientific consensus is nothing beyond; "could be" so don't tell our children that science "believes" as much as you do.

Next time know exactly what it is you are "believing" in; 31 years of "could be" and never "will be".

*Find us one scientist that can say a crisis is as real as they can say comet hits, evolution and smoking causing cancer are.

*Find us one single IPCC warning that says; "will be" and anything beyond; "could be".

*Find us one single scientist that has ever said; "inevitable" or "eventual" or "unavoidable" like they love to say comet hits are.

It wasn't a crime for science to say "maybe" for 31 years but it is a crime for you remaining "believers" to tell kids that science "believes" as much as you do.

"Between 1C and
2C increases in global mean temperatures most species, ecosystems and
landscapes will be impacted and adaptive capacity will become limited. With the
already ongoing high rate of climate change, the decline in biodiversity will
therefore accelerate and simultaneously many ecosystem services will become
less abundant."

"…analysis suggests
that despite high-level statements to the contrary, there is now little to no
chance of maintaining the global mean surface temperature at or below 2
(degrees) C (Celsius).
Moreover, the impacts associated with 2C have been revised upwards,
sufficiently so that 2C now more appropriately represents the threshold between
‘dangerous’ and ‘extremely dangerous’ climate change."

Notice how the sky is always going to fall way out in the future so
we better let these dishonest control freak Marxists create regulations
and endless new taxes now? Remember in 1973 when they claimed that there
would be no oil left in 30 years? It's more than 40 years later. These
are the same nut-cases who plant fake evidence of endangered species in
national forests.

Here is the online statement that
more than 30,000 people with advanced degrees in hard sciences have
signed. Among them are more than 9,000 Ph D's and a handful of Nobel
Laureates...

“The proposed limits on greenhouse gases
would harm the environment, hinder the advance of science and technology, and
damage the health and welfare of mankind.

“There is no convincing scientific
evidence that human release of carbon dioxide, methane, or other greenhouse
gases is causing or will, in the foreseeable future, cause catastrophic heating
of the Earth’s atmosphere and disruption of the Earth’s climate. Moreover, there is substantial scientific evidence that
increases in atmospheric carbon dioxide produce many beneficial effects upon
the natural plant and animal environments of the Earth.”

@Richard Nygaard Actually, most of the CO2 comes from burning of fossil fuels. Yeah, the stuff we use for electricity. So, rather than stop breathing, how about we stop using fossil fuels?

Also, we may be warming from the last ice age, but compared to previous warming periods, this one is significantly larger, which means that either we are warming more than usual (unlikely), or we are really screwing this earth up.

@John Deitchman Being a geologist or computer programmer does not do away with the need for you to provide proper scientific evidence to contest the claims you believe are false. Your feelings, belief & intuition, unfortunately carry no weight.

@Joe Murray Thank you! You have some sense! An interesting fact that supports your view (as well as mine) is that the campaign (propaganda) contributions that corporations flood into washington. Out of the 100 members of the senate there are approximately 70 that do believe in global warming and the "Human impact" the other 30 people say it isnt real or is a "natural problem' that humans have no control of. But if you look into statistics the average contribution by fuel-dependent industries/companies to people that do not "believe" in human caused climate change is approximately $600,000. Compared to the other 70 members of the senate that average contributions from fuel dependent industries that totals to $170,000 per person. I would say this is the "debate" that is created.

Paul M : Climate scientists say "could" be because there is no way to know exactly how much CO2 we'll emit in the coming years. The IPCC, however, has said that if we keep burning carbon at our present rate, 37 trillion tons of excess CO2 last year, another new record, we'll have a 5C increase in 80 years. A 3C increase would mean large scale agriculture would be impossible and 5C would be life threatening for most plant and animal species, including humans.

@Scout Anonymous One of the signatures on the infamous Oregon Petition is from my former roommate's cat. The roommate received the original junk mail, on fake National Academy of Sciences letterhead, sent to a name he used only once when he attended a pharmaceuticals trade show to collect swag. We wanted to see if the Oregon Petition project would make any attempt to validate the identities of the signers. Obviously, they didn't. That's your "30,000 people with advanced degrees in hard sciences."

@Scout Anonymous An appeal to authority is a logical fallacy. Beyond that however, this petition does not represent the scientific consensus. Consider the following paper Anderegg, William RL, et al. "Expert credibility in climate change." Proceedings of the National Academy of Sciences 107.27 (2010): 12107-12109. With a bit of searching on any academic search engine you will find many, many more such papers that show a clear consensus among those who actually understand climate. That petition you cite requires only that the signer have a BS in a relevant field, hardly the level of qualification I trust. The idea that scientists do not think global warming is happening is simply not true, and those of you that think otherwise should really check out the peer-reviewed research!